Traditional fundamental control of a pump station comprising one or more pumps is based on a pump being activated when a start condition is satisfied and is switched off when a stop condition is satisfied. Usually, there is a level instrument arrangement that detects when a pump start liquid level in the sump of the pump station is reached as well as when a pump stop liquid level is reached. According to law and custom, pump stations are almost always equipped with at least two pumps arranged in parallel, where a secondary pump just is a security in case the primary pump breaks or if the inflow to the pump station for the moment is unusually high.
Some manufacturers/users only use the primary pump in normal pumping, but this gives a large wear of the primary pump at the same time as the disposal of faultless function of the secondary pump is uncertain when the same is indeed needed. On the contrary, it is more common to alternate between the primary pump and the secondary pump when emptying of the sump is required.
A simple way of alternation, in view of control, includes that the pumps are active every second time, another way of alternation is to let them be active equally long as measured over a certain time, a third way to alternate the activation of the pumps is to let the pumps be active, for instance, every second day. However, all of said ways of alternation require that the control unit of the pump station, or the respective control unit of the pumps, has knowledge about the number of pumps that are arranged in the pump station and/or that communication takes place between the pumps.
One way to try to avoid communication between the pumps is shown in U.S. Pat. No. 7,195,462, wherein each one of several pumps of one and the same pump station has at least two predefined pump start liquid levels, and wherein the pump that was active most recently assumes the higher pump start liquid level and the other pumps keep the lower pump start liquid level, with the purpose of one of the most recently inactive pumps instead of the most recently active pump being to be activated the next time the liquid level in the sump rises sufficiently high. However, this publication shows that each pump has to be aware of how many other pumps being arranged in the sump.
Common to the previously known solutions is that, at least as regards waste water applications, a considerable tidal mark of grease and dirt will be built up at the fixed pump start liquid level, which is not desirable.